The giant dielectric behavior of CaCu
3
Ti
4
O
12
(CCTO) has been widely investigated owing to its potential applications in electronics; however, the loss tangent (tan
δ
) of this material is too ...large for many applications. A partial substitution of CCTO ceramics with either Al
3+
or Ta
5+
ions generally results in poorer nonlinear properties and an associated increase in tan
δ
(to ~0.29–1.15). However, first-principles calculations showed that self-charge compensation occurs between these two dopant ions when co-doped into Ti
4+
sites, which can improve the electrical properties of the grain boundary (GB). Surprisingly, in this study, a greatly enhanced breakdown electric field (~200–6588 V/cm) and nonlinear coefficient (~4.8–15.2) with a significantly reduced tan
δ
(~0.010–0.036) were obtained by simultaneous partial substitution of CCTO with acceptor-donor (Al
3+
, Ta
5+
) dopants to produce (Al
3+
, Ta
5+
)-CCTO ceramics. The reduced tan
δ
and improved nonlinear properties were attributed to the synergistic effects of the co-dopants in the doped CCTO structure. The significant reduction in the mean grain size of the (Al
3+
, Ta
5+
)-CCTO ceramics compared to pure CCTO was mainly because of the Ta
5+
ions. Accordingly, the increased GB density due to the reduced grain size and the larger Schottky barrier height (
Φ
b
) at the GBs of the co-doped CCTO ceramics were the main reasons for the greatly increased GB resistance, improved nonlinear properties, and reduced tan
δ
values compared to pure and single-doped CCTO. In addition, high dielectric constant values (
ε
′ ≈ (0.52–2.7) × 10
4
) were obtained. A fine-grained microstructure with highly insulating GBs was obtained by Ta
5+
doping, while co-doping with Ta
5+
and Al
3+
resulted in a high
Φ
b
. The obtained results are expected to provide useful guidelines for developing new giant dielectric ceramics with excellent dielectric properties.
Abstract
A modified sol-gel method was used to successfully produce Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
ceramics with high dielectric permittivity. The dielectric permittivity of Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
...ceramics reaches values larger than 10
4
at room temperature and 1 kHz. Moreover, these ceramics exhibit two distinct thermally induced dielectric relaxations over a broad temperature range. The loss tangent is indeed small, ~0.032–0.035. At low temperatures, dielectric relaxation was attributed to the oxygen vacancy effect, while at high temperatures, it was attributed to grain boundary and sample-electrode contact effects. Our calculations revealed that Y and Na ions are likely to occupy Ca and Cu sites, respectively. As a result, other Cu related phases, especially CuO, were observed at the grain boundaries. Based on our analysis, there is a charge compensation between Na and Y ions in Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
. Additionally, the Cu
+
and Ti
3+
states observed in our XPS study originate from the presence of an oxygen vacancy in the lattice. Last, the primary cause of the enormous dielectric permittivity of Na
1/2
Y
1/2
Cu
3
Ti
4
O
12
ceramics primarily comes from the internal barrier layer capacitor effect.
The effects of charge compensation on dielectric and electrical properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics (x = 0−0.05) prepared by a solid-state reaction method were studied based on the ...configuration of defect dipoles. A single phase of CaCu3Ti4O12 was observed in all ceramics with a slight change in lattice parameters. The mean grain size of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics was slightly smaller than that of the undoped ceramic. The dielectric loss tangent can be reduced by a factor of 13 (tanδ ~0.017), while the dielectric permittivity was higher than 104 over a wide frequency range. Impedance spectroscopy showed that the significant decrease in tanδ was attributed to the highly increased resistance of the grain boundary by two orders of magnitude. The DFT calculation showed that the preferential sites of Al and Nb/Ta were closed together in the Ti sites, forming self-charge compensation, and resulting in the enhanced potential barrier height at the grain boundary. Therefore, the improved dielectric properties of CaCu3Ti4-x(Al1/2Ta1/4Nb1/4)xO12 ceramics associated with the enhanced electrical properties of grain boundaries. In addition, the non-Ohmic properties were also improved. Characterization of the grain boundaries under a DC bias showed the reduction of potential barrier height at the grain boundary. The overall results indicated that the origin of the colossal dielectric properties was caused by the internal barrier layer capacitor structure, in which the Schottky barriers at the grain boundaries were formed.
The effects of the sintering conditions on the phase compositions, microstructure, electrical properties, and dielectric responses of TiO2-excessive Na1/2Y1/2Cu3Ti4.1O12 ceramics prepared by a ...solid-state reaction method were investigated. A pure phase of the Na1/2Y1/2Cu3Ti4.1O12 ceramic was achieved in all sintered ceramics. The mean grain size slightly increased with increasing sintering time (from 1 to 15 h after sintering at 1070 °C) and sintering temperature from 1070 to 1090 °C for 5 h. The primary elements were dispersed in the microstructure. Low dielectric loss tangents (tan δ~0.018–0.022) were obtained. Moreover, the dielectric constant increased from ε′~5396 to 25,565 upon changing the sintering conditions. The lowest tan δ of 0.009 at 1 kHz was obtained. The electrical responses of the semiconducting grain and insulating grain boundary were studied using impedance and admittance spectroscopies. The breakdown voltage and nonlinear coefficient decreased significantly as the sintering temperature and time increased. The presence of Cu+, Cu3+, and Ti3+ was examined using X-ray photoelectron spectroscopy, confirming the formation of semiconducting grains. The dielectric and electrical properties were described using Maxwell–Wagner relaxation, based on the internal barrier layer capacitor model.
In this work, the colossal dielectric properties and Maxwell—Wagner relaxation of TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 (x = 0–0.2) ceramics prepared by a solid-state reaction method are investigated. A ...single phase of Na1/2Y1/2Cu3Ti4O12 is achieved without the detection of any impurity phase. The highly dense microstructure is obtained, and the mean grain size is significantly reduced by a factor of 10 by increasing Ti molar ratio, resulting in an increased grain boundary density and hence grain boundary resistance (Rgb). The colossal permittivities of ε′ ~ 0.7–1.4 × 104 with slightly dependent on frequency in the frequency range of 102–106 Hz are obtained in the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics, while the dielectric loss tangent is reduced to tanδ ~ 0.016–0.020 at 1 kHz due to the increased Rgb. The semiconducting grain resistance (Rg) of the Na1/2Y1/2Cu3Ti4+xO12 ceramics increases with increasing x, corresponding to the decrease in Cu+/Cu2+ ratio. The nonlinear electrical properties of the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics can also be improved. The colossal dielectric and nonlinear electrical properties of the TiO2–rich Na1/2Y1/2Cu3Ti4+xO12 ceramics are explained by the Maxwell–Wagner relaxation model based on the formation of the Schottky barrier at the grain boundary.
•We acquired very high dielectric permittivity and a considerably low loss tangent.•DFT shows that Cd2+ replaces Cu2+ sites in CaCu3Ti4O12 and Ca2+ sites in CaTiO3.•CdF2 insulating clusters only ...formed in CaCu3Ti4O12 lattice.•Doping CdF2 into CaCu3Ti4O12/CaTiO3 improves the total grain boundary capacitance.•This colossal dielectric property is consistent with the IBLC model.
Improved dielectric properties can be achieved in a binary-phase system of CaCu3Ti4O12/CaTiO3 by doping it with CdF2 to produce Ca2-xCdxCu2Ti4O12-2yF2y (x = y = 0, 0.05, 0.10, and 0.15). These ceramics were fabricated using a solid-state reaction method. A composite phase between CaCu3Ti4O12/CaTiO3 at a ratio ∼ 66.7/33.3 mol% was found in Ca2-xCdxCu2Ti4O12-2yF2y samples. The grain size of Ca2-xCdxCu2Ti4O12-2yF2y decreases with increasing dopant concentration. Interestingly, the dielectric permittivity of Ca2-xCdxCu2Ti4O12-2yF2y ceramics is more than doubled from 3446 → 7461 by increasing CdF2 levels from x = y = 0 to x = y = 0.15, while its loss tangent is lower than 0.03. Dielectric permittivity was found to be unaltered over a broad temperature range. According to density functional theory (DFT) analysis, Cd2+ is more likely to occupy the Cu2+ sites in the CaCu3Ti4O12 phase, while Cd2+ replaces Ca2+ ions in the CaTiO3 phase. Impedance spectroscopy findings suggest that the principal source of the tremendous dielectric response in this binary-phase system is an internal barrier layer capacitor composed of semiconducting grains and insulating grain boundaries. According to XPS analysis, electronic charge hopping between Cu+↔Cu2+ and Ti3+↔Ti4+ could be a primary source of n-type semiconducting grains of the CaCu3Ti4O12 phase.
In this study we employed the B3LYP/6-311++G(d,p) method combined with the CIS/6-311++G(d,p) calculation to investigate the effects of the type and the number of alkali metal atoms (Li, Na, K) on the ...geometric, electronic, and optical properties of alkali metals substituted into adamantanes. Substituting alkali metal (Li, Na, K) atoms caused significant changes in the electronic and optical properties of adamantane. The Ad-1Li, Ad-1Na, and Ad-1K structures showed a dramatically decreased energy gap and ionization potential, while adding more alkali metal atoms slightly decreased these properties. Substituting more alkali metals led to a shift in the maximum absorption wavelength from the visible to the infrared region, depending on the type of alkali metal atom substituted. The magnitude of shift occurred in the following order: Li < Na < K. These characteristics suggest the possibility of tunable electronic structures of this material for optoelectronic device applications.
Geometric, electronic and optical properties of alkali metal (Li, Na, K) substituted adamantanes have been investigated by means of first principles calculations. In this study we employed the B3LYP/6-311++G(d,p) method combined with the CIS/6-311++G(d,p) calculation to investigate the effects of the type and the number of alkali metal atoms (Li, Na, K) on the geometric, electronic, and optical properties of alkali metals substituted into adamantanes. Substituting alkali metal (Li, Na, K) atoms caused significant changes in the electronic and optical properties of adamantane. Display omitted
•Hybrid QM/MM calculations were employed for investigating monohydration and polyhydration of the sorbitan monostearate (Span 60) head group in the gas phase.•The hydration energies, structural and ...electronic properties of the Span 60 head group were examined.•Adding more water molecules did not significantly affect the hydration energy of Span 60 because of steric hindrance and electrostatic repulsions between water molecules.•Interactions between the ether oxygen of a Span 60 head group and water molecules are still prominent for all hydration complexes.
The binding interactions and structural and electronic properties involved in microhydration of Span 60•(H2O)n (n = 1–10) complexes were investigated using hybrid QM/MM calculations in the gas phase. The Span 60 head group and its hydrating water molecules were treated using the QM method, while the Span 60 tail group was treated with the MM method. For the monohydration complex, it was revealed that the hydration energy and binding force are strongly dependent on the hydration sites (O1–O6) of the Span 60 head group. The ether oxygen (O4) exhibited the highest hydration energy and the strongest hydrogen bonds. An ester oxygen atom (O2) showed a lower binding force due to its delocalized lone pair electrons. For polyhydration complexes, it was found that the average hydration energy of each polyhydration complex is close to that of the monohydrated form because of steric hindrance and electrostatic repulsions between water molecules. Interaction between the ether oxygen (O4) and water molecules is still prominent for all polyhydration complexes. The charge transfer phenomenon is noticeably altered when a few water molecules bind with a particular oxygen atom of the Span 60 head group, leading to an increase in the polarity of this group. Hydrogen bond lengths tend to decrease slightly and the conformation change of the Span 60 head group is significantly affected as the number of hydrated water molecules increases.
Hybrid QM/MM calculations have been employed for investigating monohydration and polyhydration of a sorbitan monostearate (Span 60) head group in the gas phase. Binding interactions and molecular properties of the hydration complexes were examined. The ether oxygen of the Span 60 head group exhibited the highest hydration energy and the strongest hydrogen bonds. The hydration energy of the Span 60 head group is slightly affected as the number of hydrated water molecules increases. However, the conformation of the Span 60 head group has changed significantly. Display omitted
Molecular dynamics simulations were carried out to investigate structural and dynamical properties of Span60 bilayers with and without cholesterol inclusion. This study reveals that in the presence ...of cholesterol the Span60 bilayer shows loosely molecular packing and higher mobility. Without cholesterol, Span60 bilayer is closely packed with stronger Span60/Span60 interaction.
•Molecular structure and dynamical properties of the Span60 bilayers with and without cholesterol inclusion have been first investigated by using molecular dynamics simulations.•Addition of cholesterol to the Span60 bilayer strongly influence the molecular self-assembly and dynamic behavior.•The Span60 bilayer with cholesterol exhibit more fluidity and stability.•Hydrogen bonds play a crucial role for the Span60 bilayer stability.
Niosomes are non-ionic surfactant vesicles having a bilayer structure formed by self-assembly of hydrated surfactants, usually with cholesterol incorporation. Stability and mechanical properties of niosomes strongly depend on type of non-ionic surfactants and compositions used. In this study we present the structural and dynamical properties of niosome bilayers composed of sorbitan monostearate (Span60) with 0% and 50% cholesterol compositions which are investigated by using molecular dynamics simulations. The simulations reveal that niosome bilayer without cholesterol prefer to form in the gel phase with a higher order structure, while in the presence of cholesterol the bilayer exhibits more fluidity having a less ordered structure. The niosome bilayer with 50% cholesterol inclusion shows an increase of area per lipid (∼11%) and thickness (∼39%) compared with the niosome bilayer without cholesterol. The Span60 tailgroup orientation of the niosome bilayers without cholesterol exhibits more tilt (34.5o±0.5) than that of the bilayer with 50% cholesterol (15.4o±0.8). Additionally, our results show that the addition of cholesterol to the bilayer causes the higher in lateral and transverse diffusion, as well as an increase in the hydrogen bond number between Span60 and water. Such characteristics not only enhance the niosome stability but also increase the fluidity, which are necessary for the niosomal drug delivery.
•Very high ε′ of CaCu3-xMgxTi4O12 ranging from 6.58 × 103 to 2.87 × 104 was observed.•Low loss tangents of ∼0.011 − 0.025 were obtained in these ceramics.•ε′ of CaCu2.90Mg0.10Ti4O12 is stable over ...temperature range from −60 to 120 °C.•Vo was likely to be isolated from the Mg atoms in the CaCu3-xMgxTi4O12.•Colossal dielectric properties support the internal barrier layer capacitor model.
In this work, CaCu3-xMgxTi4O12 (x = 0, 0.05, and 0.10) ceramics were successfully prepared via a chemical combustion method. No impurity phase was detected in these two ceramics. Fine−grained microstructures were formed in all CaCu3-xMgxTi4O12 ceramics. Interestingly, high dielectric permittivities of ∼6.58 × 103 − 2.87 × 104 with low loss tangents of ∼0.011 − 0.025 were obtained in the Mg-doped CaCu3Ti4O12 ceramics. According to the electrical measurements, improved dielectric properties in the doped ceramic samples were a result of an enhanced grain boundary response. To elucidate the electronic structure of CaCu2.90Mg0.10Ti4O12, first-principles calculations were carried out. It was found that two Mg atoms preferentially interact. Moreover, by adding an oxygen vacancy into the CaCu2.90Mg0.10Ti4O12 structure, our results revealed that it was likely to be isolated from the Mg atoms, indicating an oxygen loss suppression during the sintering process. This resulted in an enhanced grain boundary resistance in the CaCu2.90Mg0.10Ti4O12 ceramic. Based on both the experimental and computational studies, the internal barrier layer capacitor (IBLC) model was found to be the primary origin of the colossal dielectric response in all CaCu3-xMgxTi4O12 ceramics examined in the current study.
Graphical Abstract
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